1. Field of the Invention
The present invention relates to a method of making a camshaft for use in a valve mechanism of an internal combustion engine, and more particularly to a process for providing a camshaft of the type having a cam piece composed of inner and outer layers.
2. Description of the Prior Art
It is well known how to make a camshaft of the type having a fitting member composed of at least two layers each being made from an individual sintering materials.
In JP A 58-41211, there is disclosed a cam lobe composed of three layers of which the inner layer is formed out of a steel sleeve and welded to the shaft. The intermediate and outer layers are respectively made from sintering materials which shrink and yield a liquid phase when sintered. In JP U 58-191458, there is disclosed a cam piece composed of three layers of which the outer and inner layers are made from different liquid phase sintering materials, the intermediate layer being made from a solid phase sintering material. The cam piece is joined with a steel shaft by sintering in which the inner layer yields a liquid phase. In JP U 58-192942, there is disclosed a journal composed of two layers of which the inner layer is made from a liquid phase sintering material, the outer layer being made from a solid phase sintering material. In JP A 59-219402, there is disclosed a cam piece composed of two layers of which the inner layer is made from a liquid phase sintering material, the outer layer being made from an iron base sintering material. In JP A 60-39105, there is disclosed a cam piece composed of two layers. The inner layer is made from a copper-base sintering material while the outer layer is made from an iron-base sintering material. In JP A 60-43405, there is disclosed a cam lobe composed of two layers. The inner layer has a shrinkage of less than 2% while the outer layer has a shrinkage of more than 2%. In JP A JP A 59-155660, there is disclosed a cam piece which is composed of two layers of which the outer layer is made from an anti-wear alloy sintering material. The inner layer is made from high toughness sintering material or weld alloy and bonded to a shaft by bulging.
There are disadvantages in the prior art: The cam lobe as shown in JP A 58-41211 requires a process for welding the inner layer of steel sleeve to the shaft, thereby increasing the cost of production. The cam piece as shown in JP A 58-41211 or JP U 58-191458 disadvantageously needs complicated and expensive dies because of being composed of three layers made from sinterable materials. The cam piece of JP U 58-191458 is not reliable in bonding between the outer layer and intermediate layer, since the bonding between the solid and liquid phase sintered materials is less reliable at their common boundary surface. This means that the journal as shown in JP U 58-192942 is less reliable in bonding at the common boundary surface between the solid and liquid phase sintered materials. The cam piece as shown in JP A 59-219402 is also less reliable in bonding at the boundary surface between the outer and inner layers, since the bonding between the iron-base and liquid phase sintered materials is less reliable at their common boundary surface. The method as shown in JP A 60-39105 is easy to have a failure rate beyond a tolerance limit. The reason for this is that the outer layer is caused to swell due to an early penetration of copper from the inner layer into the outer layer when sintered. The inner layer of the cam lobe as shown in JP A 60-43405 has a shrinkage of less than 2%, which is too small to make a reliable bonding between the cam lobe and the shaft. The camshaft as shown in JP A 59-155660 is disadvantageously limited in shape, because the cam piece is bonded to the shaft by bulging.